This application is an application under 35 U.S.C. Section 371 of International Application Number PCTJFR00/02338 filed on Aug. 18, 2000.
The present invention relates to a method of grindlessly emulsifying a silicone in water.
The emulsification methods most commonly used in the art are methods which involve mixing an aqueous phase and an oily phase with grinding, i.e., under high shear. High-shear conditions are generated, for example, by stirring in a colloidal grinder.
The major drawback of these methods is that they make it difficult if not impossible to produce heat-sensitive formulations in the form of emulsions.
The high shear involved in the methods of emulsification with grinding causes heating of the phases present and subjects their constituents to high temperatures. For example, when the oily phase comprises an organic solvent in addition to the silicone, the heating of the oily phase may cause the surfactant to dissolve in the oily phase, thereby promoting the formation of an inverse emulsion of water-in-oil type. In the context of the invention, however, the aim is to prepare direct emulsions of the oil-in-water type.
By heat-sensitive formulation is meant a formulation comprising one or more heat-sensitive constituents.
Within the context of the invention, a heat-sensitive constituent is a compound which is inherently temperature sensitive or is made temperature sensitive by its combination with other compounds that are present in the formulation.
Within the art, a distinction is made between direct emulsification methods and inverse emulsification methods. The direct emulsification methods for obtaining emulsions of oil-in-water type are methods in which, throughout its preparation and up until the time of its formation, the emulsion is a direct emulsion, i.e., an oil-in-water emulsion.
The inverse emulsification methods for obtaining emulsions of oil-in-water type involve the initial formation of a water-in-oil emulsion (commonly designated an inverse emulsion), then the inversion of this emulsion, whereby an oil-in-water emulsion is obtained.
The direct emulsification methods are not suitable for the industrial-scale preparation of emulsions with a high oil concentration.
In this case, in effect, the oily phase must be added slowly to the aqueous phase, this being undesirable from the standpoint of operating deadlines.
The inverse emulsification methods that are known in the art do not all lead to the preparation of fine, monodisperse emulsions.
The method of the invention aims to solve all of these problems by allowing the emulsification of viscous or slightly viscous oils in the presence where appropriate of one or more heat-sensitive constituents, with control of the particle size and of the polydispersity. The emulsions obtained by this method may have a very high oil concentration and may exhibit a very high weight ratio of oil to surfactant, of more than 9/1, for example.
More specifically, the method of the invention, which is a method of grindlessly emulsifying a silicone in water, comprises the steps of:
a) preparing a primary emulsion of oil-in-water type comprising said surfactant and said silicone under a shear of less than 100 sxe2x88x921 by adding an aqueous phase to an oily phase comprising said silicone, the proportion of oily phase in the primary emulsion being less than the maximum proportion Pmax above which addition of the aqueous phase to the oily phase does not make it possible to prepare an emulsion of oil-in-water type, and the weight ratio of surfactant to water in the primary emulsion being such that a mixture in this same ratio of the water and the surfactant leads to an organized phase to the exclusion of an inverse phase;
b) enriching the emulsion thus prepared with an oily enrichment phase comprising said silicone by mixing said emulsion with said oily enrichment phase under a shear of less than 100 sxe2x88x921, the amount of oily enrichment phase being less than the maximum amount above which addition of said emulsion to said oily phase does not make it possible to prepare an emulsion of oil-in-water type, thereby giving an emulsion of oil-in-water type; and
c) where appropriate, repeating the implementation of step b) one or more times until the desired concentration of surfactant and/or silicone in the final emulsion is obtained and/or until the desired particle size is obtained.
The silicones which can be emulsified by the method of the invention are, for example, polyorganosiloxane oils, gums or resins.
Among the polyorganosiloxane oils and gums which can be employed, mention may be made of those consisting of units and formula:
Rxe2x80x23-aRaSiO1/2 and R2SiO
in which formulae:
a is an integer from 0 to 3
the radicals R are identical or different and represent:
a saturated or unsaturated aliphatic hydrocarbon group containing from 1 to 10 carbon atoms;
an aromatic hydrocarbon group containing from 6 to 13 carbon atoms;
a polar organic group bonded to the silicon by a Sixe2x80x94C or Sixe2x80x94Oxe2x80x94C bond;
a hydrogen atom:
the radicals Rxe2x80x2 are identical or different and represent
an OH group
an alkoxy or alkenyloxy group containing from 1 to 10 carbon atoms;
an aryloxy group containing from 6 to 13 carbon atoms;
an acyloxy group containing from 2 to 13 carbon atoms;
a ketiminoxy group containing from 3 to 8 carbon atoms;
an amino- or amido-functional group containing from 1 to 6 carbon atoms and bonded to the silicon by a Sixe2x80x94N bond;
preferably at least 80% of the radicals R of said oils representing a methyl group.
Among the polyorganosiloxane resins which can be employed, mention may be made of those consisting of units of formulae:
RSiO3/2 (T unit) and/or SiO2 (Q unit)
in association with units of formula:
Rxe2x80x23-aRaSiO1/2 (M unit) and/or R2SiO (D unit)
in which formulae a, R and Rxe2x80x2 are as defined above.
These resins are generally of the MQ, MDQ, TDM, TD or MT type.
As examples of aliphatic or aromatic hydrocarbon radicals R, mention may be made of the following groups:
alkyl, preferably optionally halogenated C1-C10 alkyl, such as methyl, ethyl, octyl or trifluoropropyl;
alkoxyalkylene, preferably C2-C10, more preferably C2-C6, such as xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH3;
xe2x80x94CH2xe2x80x94CH2xe2x80x94Oxe2x80x94CH3;
alkenyls, preferably C2-C10 alkenyl, such as vinyl, allyl, hexenyl, decenyl or decadienyl;
alkenyloxyalkylene such as xe2x80x94(CH2)3xe2x80x94Oxe2x80x94CH2xe2x80x94CHxe2x95x90CH2, or alkenyloxyalkoxyalkyl such as xe2x80x94(CH2)3xe2x80x94OCH2xe2x80x94CH2xe2x80x94Oxe2x80x94CHxe2x95x90CH2, in which the alkyl moieties are preferably C1-C10 and the alkenyl moieties are preferably C2-C10;
aryls, preferably C6-C13, such as phenyl.
As examples of polar organic groups R, mention may be made of the following groups:
hydroxy-functional groups such as alkyl groups substituted by one or more hydroxyl or di(hydroxyalkyl)amino groups and optionally interrupted by one or more divalent hydroxyalkylamino groups. By alkyl is meant a preferably C1-C10, more preferably C1-C6, hydrocarbon chain;
examples of these groups are xe2x80x94(CH2)3xe2x80x94OH;
xe2x80x94(CH2)4N(CH2CH2OH)2;
xe2x80x94(CH2)3xe2x80x94N(CH2CH2OH)xe2x80x94CH2xe2x80x94CH2xe2x80x94N(CH2CH2OH)2:
amino-functional groups such as alkyl substituted by one or more amino or aminoalkylamino groups in which alkyl is as defined above; examples thereof are xe2x80x94(CH2)3xe2x80x94NH2; (CH2)3xe2x80x94NHxe2x80x94(CH2)2NH2;
amido-functional groups such as alkyl substituted by one or more acylamino groups and optionally interrupted by one or more divalent alkyl-COxe2x80x94N less than  groups in which alkyl is as defined above and acyl represents alkylcarbonyl; one example is the group xe2x80x94(CH2)3xe2x80x94N(COCH3) xe2x80x94(CH2)2NH(COCH3);
carboxy-functional groups such as carboxyalkyl optionally interrupted by one or more oxygen or sulfur atoms, in which alkyl is as defined above; one example is the group xe2x80x94CH2xe2x80x94CH2xe2x80x94Sxe2x80x94CH2xe2x80x94COOH.
As examples of radicals Rxe2x80x2, mention may be made of the following groups:
alkoxy, preferably C1xe2x80x94C10, more preferably C1-C6, such as methoxy, ethoxy or octyloxy;
alkenyloxy, preferably C2-C10, more preferably C2-C6;
aryloxy, preferably C6-C13, such as phenyloxy;
acyloxy in which acyl is preferably (C1-C12)alkylcarbonyl such as acetoxy;
ketiminoxy containing from 3 to 8 carbon atoms, such as ONxe2x95x90C(CH3)C2H5;
amino-functional groups such as alkyl or aryl substituted by amino, alkyl being preferably C1-C6 and aryl designating a cyclic aromatic hydrocarbon group which is preferably C6-C13, such as phenyl; examples thereof are ethylamino and phenylamino;
amido-functional groups such as alkylcarbonylamino in which alkyl is preferably C1-C6; examples thereof are methylacetamido.
Specific examples of xe2x80x9cD unitsxe2x80x9d that may be mentioned include: (CH3)2SiO; CH3(CHxe2x95x90CH2)SiO; CH3(C6H5)SiO; (C6H5)2SiO; CH3HSiO; CH3(CH2xe2x80x94CH2xe2x80x94CH2OH)SiO.
Specific examples of xe2x80x9cM unitsxe2x80x9d that may be mentioned include: (CH3)3SiO1/2; (CH3)2(OH)SiO/1/2; (CH3)2(CHxe2x95x90CH2)SiO1/2; (CH3)2HSiO1/2:(OCH3)3SiO1/2:[Oxe2x80x94C(CH3)xe2x95x90CH2]3SiO1/2:[ONxe2x95x90C (CH3)]3SiO1/2; (NHxe2x80x94CH3)3SiO1/2; (NHxe2x80x94COxe2x80x94CH3)3SiO1/2.
Specific examples of xe2x80x9cT unitsxe2x80x9d that may be mentioned include: CH3SiO3/2; (CHxe2x95x90CH2)SiO3/2; HSiO3/2.
When said oils, gums or resins contain reactive and/or polar radicals R (such as H, OH, vinyl, allyl, hexenyl, aminoalkyls, etc), the latter generally represent not more than 2% of the weight of the oil or gum and not more than 10% of the weight of the resin.
The viscous polydimethylsiloxane and xcex1,xcfx89-bis(hydroxy)polydimethylsiloxane oils and also the polydimethylsiloxane, polyphenylmethylsiloxane and xcex1,xcfx89-bis(hydroxy)polydimethylsiloxane gums are well-known commercial products.
The viscous DT or MDT polymethylsiloxane resins containing from 1 to 5% by weight of silanol functions are also commercial products.
Preference is given more particularly to xcex1,xcfx89-bis(trimethyl)polydimethylsiloxane oils and gums; xcex1,xcfx89-bis(hydroxy)polydimethylsiloxane oils and gums; hydroxylated polydimethylsiloxane resins of type DT or MDT, and their mixtures in any proportions.
The silicones which can be emulsified by the method of the invention are very variable in viscosity. A distinction is made between those of high viscosity, whose viscosity is greater than 1 000 mPaxc2x7s, greater than 3 000 mPaxc2x7s for example, preferably greater than 5 000 mPaxc2x7s, and may attain values greater than 10 000 mPaxc2x7s and, for example, 500 000 mPaxc2x7s, and those of low viscosity whose viscosity is less than 1 000 mPaxc2x7s, and preferably between 1 and 1 000 mPaxc2x7s, for example between 20 and 1 000 mPaxc2x7s.
Within the context of the invention, the viscosity values are values of the dynamic viscosities measured at 25xc2x0 C. using a Brookfield viscometer in accordance with the indications in the standard AFNOR NFT 76102.
Preferred examples of polydimethylsiloxane oils are those whose polydimethylsiloxane chain is blocked at its two ends by a (CH3)3SiO1/2 or (CH3)2 (OH)SiO1/2 group, of dynamic vicosity 20 mPaxc2x7s, 350 mPaxc2x7s, 750 mPaxc2x7s, 80 000 mPaxc2x7s or 135 000 mPaxc2x7s.
Preferred examples of polymethylsiloxane resins are:
a hydroxylated polymethylsiloxane resin of type MDT having 0.5% by weight of hydroxyl groups and consisting of 62% by weight of CH3SiO3/2 units, 24% by weight of (CH3)2SiO1/2 units and 14% by weight of (CH3)3SiO1/2 units; and
a hydroxylated polymethylsiloxane resin of type DT having 2.2% by weight of hydroxyl groups and consisting of 69% by weight of CH3SiO3/2 units and 31% by weight of (CH3)2SiO2/2 units.
When implementing each of steps a), b), and c) of the method of the invention, it is essential that the temperature remains less than 80xc2x0 C., preferably less than 60xc2x0 C., more preferably less than 35xc2x0 C., for example, between 15 and 35xc2x0 C.
An equally important parameter is the intensity of shearing. The emulsification proceeds via steps of mixing the various phases present. It is during these mixing steps that the shear must remain less than 100 sxe2x88x921. This essential characteristic distinguishes the method of the invention from a method with grinding.
So as to control the shear, the various phases are stirred in appropriate mixers such as mixtures with counterrotational stirring (for example, trimix from the Rayneri company), planetary mixers or mixtures with central stirring of the anchor or butterfly type.
The nature of the surfactant which can be used for emulsification is not critical according to the invention. It is therefore possible to use nonionic, cationic, anionic or even zwitterionic surfactants.
Examples of anionic surfactants include:
alkyl ester sulfonates of formula Rxe2x80x94CH(SO3M)xe2x80x94COORxe2x80x2 in which R represents a C8-C20, preferably C10-C16, alkyl radical, Rxe2x80x2 a C1-C6, preferably C1-C3, alkyl radical, and M an alkali metal cation (sodium, potassium or lithium), a substituted or unsubstituted ammonium cation (methyl-, dimethyl-, trimethyl-, tetramethylammonium, dimethylpiperidinium, etc.) or a cation derived from an alkanolamine (monoethanolamine, diethanolamine, triethanolamine, etc.). Particular mention may be made of the methyl ester sulfonates whose radical R is C14-C16;
alkyl sulfates of formula ROSO3M in which R represents a C10-C24, preferably C12-C20 and very particularly C12-C18, alkyl or hydroxyalkyl radical, M representing a hydrogen atom or a cation of the same definition as above, and their ethoxylenated (EO) and/or propoxylenated (PO) derivatives, having on average from 0.5 to 6, preferably from 0.5 to 3, EO and/or PO units;
alkyl amide sulfates of formula RCONHRxe2x80x2OSO3M in which R represents a C2-C22, preferably C6-C20, alkyl radical, Rxe2x80x2 a C2-C3 alkyl radical, M representing a hydrogen atom or a cation with the same definition as above, and their ethoxylenated (EO) and/or propoxylenated (PO) derivatives, having on average from 0.5 to 60 EO and/or PO units;
salts of saturated or unsaturated C8-C24, preferably C14-C20, fatty acids, C9-C20 alkylbenzene-sulfonates, C8-C22 primary or secondary alkylsulfonates, alkylglycerol sulfonates, the sulfonated polycarboxylic acids described in GB-A-1 082 179, paraffin sulfonates, N-acyl-N-alkyltaurates, alkyl phosphates, alkylisethionates, alkylsuccinamates and alkylsulfosuccinates, sulfosuccinate monoesters or diesters, N-acylsarcosinates, alkylglycoside sulfates, polyethoxycarboxylates, the cation being an alkali metal (sodium, potassium or lithium), a substituted or unsubstituted ammonium residue (methyl-, dimethyl-, trimethyl-, tetramethylammonium, dimethylpiperidinium, etc.) or derived from an alkanolamine (monoethanol-amine, diethanolamine, triethanolamine, etc.); and
phosphate or alkyl phosphate esters.
Examples of nonionic surfactants include:
polyoxyalkylenated (polyethoxyethylenated, polyoxypropylenated or polyoxybutylenated) alkylphenols whose alkyl substituent is C6-C12 and which contains from 5 to 25 oxyalkylenated units; by way of example, mention may be made of Triton X-45, X-114, X-100 or X-102, which are sold by Rohm and Haas Cy.;
glucosamines and glucamides;
glycerolamides derived from N-alkylamines (U.S. Pat. No. 5,223,179 and FR-A-1,585,966);
polyoxyalkylenated C8-C22 aliphatic alcohols containing from 1 to 25 oxyalkylene (oxyethylene, oxypropylene) units; by way of example, mention may be made of Tergitol 15-S-9 and Tergitol 24-L-6 NMW, sold by Union Carbide Corp., Neodol 45-9, Neodol 23-65, Neodol 45-7 and Neodol 45-4, sold by Shell Chemical Cy., and Kyro EOB sold by The Procter and Gamble Cy.
products resulting from the condensation of ethylene oxide with a hydrophobic compound resulting from the condensation of propylene oxide with propylene glycol, such as the Pluronics sold by BASF;
amine oxides such as (C10-C18 alkyl)dimethyl-amine oxides and (C8-C22 alkoxy)ethyldihydroxyethylamine oxides;
the alkyl polyglycosides described in U.S. Pat. No. 4. 565 647 and their polyoxyalkylenated derivatives;
C8-C20 fatty acid amides;
ethoxylated fatty acids; and
ethoxylated amides, amines and amidoamines;
esters of polyoxyalkylenated or non-polyoxyalkylenated sorbitan with C8-C20 fatty acids, such as the oleates of polyoxyethylenated sorbitan (for example, SPAN and TWEEN);
esters of sucrose with C8-C20 fatty acids;
mixtures of esters of sucrose with C8-C20 fatty acids with C8-C20 fatty acid mono-, di- and/or triglycerides.
Examples of amphoteric and zwitterionic surfactants include:
those of the betaine type such as betaines, sulfobetaines, amidoalkylbetaines, sulfobetaines, alkylsultaines and alkyltrimethylsulfobetaines,
condensation products of fatty acids and protein hydrolyzates, alkylamphopropionates or
alkylamphodipropionates,
amphoteric derivatives of alkylpolyamines such as Amphionic XL sold by Rhxc3x4ne-Poulenc, Ampholac 7T/X and Ampholac 7C/X sold by Berol Nobel, and
cocoamphoacetates and cocoamphodiacetates.
The nonionic surfactants are nevertheless preferred and more particularly the nonionic surfactants of the polyoxyalkylenated linear- or branched-chain fatty alcohol type resulting from the condensation of a fatty alcohol with a C2-C4 alkylene oxide such as ethylene oxide, propylene oxide or butylene oxide. By way of example, mention may be made of ethoxylated isotridecyl alcohol.
More generally, preferred nonionic surfactants are those whose HLB is between 8 and 16, preferably between 10 and 15, more preferably between 12.5 and 15.
The term HLB (hydrophilic lipophilic balance) designates the ratio of the hydrophilicity of the polar groups of the surfactant molecules to the hydrophobicity of their lipophilic moiety. HLB values are reported in particular in various basic manuals such as the xe2x80x9cHandbook des excipients pharmaceutiques, The Pharmaceutical Press, London, 1994)xe2x80x9d.
The method of the invention comprises at least steps a) and b), step a) consisting in preparing a direct primary emulsion of oil-in-water type including the silicone and the surfactant, this primary emulsion being enriched in step b) by an oily enrichment phase based on said silicone. It should be understood that, as a result of this enrichment step, the silicone concentration of the resulting emulsion increases whereas the surfactant concentration of the resulting emulsion is reduced.
The primary emulsion prepared by adding an aqueous phase to an oily phase is a direct emulsion. In accordance with the invention, a direct emulsion means any emulsion of oil-in-water type. Inverse emulsions denote any emulsion of water-in-oil type.
In accordance with the invention, the aqueous phase comprises essentially water but preferably also the surfactant used for the emulsification.
It will be noted that the method of the invention is not limited to the emulsification of one silicone but can be used for emulsifying a number of silicones, said silicone or silicones being optionally mixed with one or more organic oils which are different than silicones but compatible with them.
Within the context of the invention, the oily phase comprises one or more silicone compounds as defined above and, optionally, one or more constituents selected from a silane, a siliceous or nonsiliceous filler, a solvent and/or an additive which is compatible or compatibilized with silicones.
As additives mention may be made of anti-UV agents, antioxidants, dyes, antibacterials, fungicides, skin whiteners, vitamins, and perfumes.
Among the solvents of the silicone oils, gums or resins, optionally present in the oily phase, mention may be made of volatile cyclic polyorganosiloxanes (for example, octamethylcyclotetra-siloxane or decamethylcyclopentasiloxane), short polydimethylsiloxane oils (viscosity less than 100 mPaxc2x7s), hexamethyldisiloxane, ketones (for example, methyl ethyl ketone), ethers (for example diethyl ether), esters (for example, isopropyl myristate or ethyl acetate), certain chlorinated or chlorofluorinated solvents (for example, methylene, chloride or chloroform) and highly branched paraffins (for example, white oils based on isoalkanes and cycloalkanes).
The silanes may in particular be byproducts of the synthesis of said polyorganosiloxane oils, gums or resins which are employed or crosslinking agents of said oils, gums or resins.
They may be represented by the formula (Rb)Si(ORxe2x80x2)4-b, in which formula b is an integer from 0 to 4, R and Rxe2x80x2 having the definition given above. They are described in particular in U.S. Pat. No. 3 294 725; U.S. Pat. No. 4 584 341; U.S. Pat. No. 4 618 642; U.S. Pat No. 4 608 412; U.S. Pat. No. 4 525 565; EP-A-340 120; EP-A-364 375; FR-A-1 248 826; FR-A-1 423 477; and EP-A-387 157.
As examples, mention may be made of the following silanes:
Si(OC2H5)4; CH3Si (OCH3)3; CH3Si(OC2H5)3; (C2H5O)3Si(OCH3); CH2xe2x95x90CHSi (OCH3)3; CH3(CH2xe2x95x90CH)Si (OCH3)2; CH2xe2x95x90CHSi (OC2H5)3; CH2xe2x95x90CHSi[ONxe2x95x90C(CH3)C2H5]3; CH3Si[ONxe2x95x90C(CH3)2]3; CH3Si[Oxe2x80x94C(CH3)xe2x95x90CH2]3; methyltris (N-methylacetamido-silane); methyltris (cyclohexylaminosiloxane).
They are generally present in amounts of the order of from 0 to 10 parts by weight, preferably of the order of from 0 to 5 parts by weight, per 100 parts by weight of polyorganosiloxane oil(s) and/or gum(s) and/or resin, when they are reaction byproducts.
When it is their function as crosslinking agents of the hydroxylated oils, gums or resins which is desired, they are generally present in amounts of the order of from 0.5 to 30 parts by weight, preferably of the order of from 2 to 8 parts by weight, per 100 parts by weight of oil(s) and/or gum(s) and/or resin(s).
Said silanes can also be an additive which makes it possible to modify the physiocochemical properties, of adhesion in particular, of the silicone compositions of diverse applications which are obtained from the aqueous emulsions prepared by the method of the invention. Examples of such silanes are described in particular in EP-A-340 120. Among this category of silanes, mentioned may be made of aminopropyltriethoxy-silane, aminopropylmethyldiethoxysilane; and glycidoxypropyltrimethoxysilane. They are employed in quantities which can range up to 200%, generally of the order of from 2 to 100% of the weight of the oil(s) and/or gum(s) and/or resin(s).
In accordance with the invention, the semireinforcing: by way of example, mention may be made of colloidal silicas, pyrogenic and precipitated silica powders, diatomaceous earths, ground quartz, natural calcium carbonate, hydrated alumina, magnesium hydroxide, carbon black, titanium dioxide, aluminum oxide, vermiculite, zinc oxide, mica, talc, iron oxide, barium sulfate, slaked lime; the particle size of these fillers is generally of the order of from 0.001 to 300 xcexcm; they are generally present in amounts which can range up to 300%, preferably of the order of from 3 to 100% of the weight of oil(s) and/or gum(s) and/or resin(s).
In accordance with the invention, the oily phase may further comprise the surfactant when the latter has not been added to the aqueous phase.
Accordingly, the surfactant required for the preparation of the primary emulsion is alternatively present in its entirety in the aqueous phase or present in its entirety in the oily phase or else distributed in any proportion between these two phases.
In yet another embodiment of the invention, the surfactant is added separately and in one go to the oily phase which is maintained at rest, at the same time as the aqueous phase.
In one preferred embodiment of the invention, the entirety of the surfactant is present within the aqueous phase before addition.
The addition of the aqueous phase to the oily phase can be carried out in any way whatsoever.
A first procedure consists in adding the aqueous phase to the oily phase, which is maintained with stirring, gradually or in small amounts, and, for example, dropwise, stirring being continued following addition until a direct emulsion is obtained by inversion.
As a variant, it is possible to add the aqueous phase, on the one hand, and, optionally, the surfactant, on the other hand (when the latter is not present in its entirety in the aqueous and oily phases) in one go to the oily phase at rest, and then to maintain the mixture with stirring until a direct emulsion is obtained.
To obtain a direct primary emulsion following addition it may be necessary to increase the intensity of stirring slightly, subject to the proviso that the shear generated must not exceed 100 sxe2x88x921. Obtaining a direct emulsion under these conditions is only possible, however, for well-defined respective proportions of surfactant, water, and organic oils.
The first condition to be observed concerns the relative proportion of water and surfactant which are present in the primary emulsion that is to be prepared. According to the invention, it is essential that the weight ratio of the water to the surfactant in the primary emulsion is such that a mixture of water and surfactant in this same ratio leads to an organized phase but not to an inverse phase. Examples of organized phases are the micellar phase, the lamellar phase, the hexagonal phase, and any mixture of two of these phases, such as a mixture of lamellar phase and micellar phase.
In accordance with the invention, it is preferred for the mixture of water and surfactant in said ratio to lead to a micellar phase, to a lamellar phase or to a mixture of these two phases.
When the silicone present in the primary emulsion is highly viscous, it is advantageous for the mixture of water and surfactant in said ratio to lead to a lamellar phase.
By xe2x80x9chighly viscousxe2x80x9d is meant more precisely a silicone with a viscosity of greater than 1 000 mPaxc2x7s, for example greater than 3000 mPaxc2x7s, preferably between 1 000 and 500 000 mPaxc2x7s, more preferably between 2 000 and 300 000 mPaxc2x7s.
When, conversely, the silicone to be emulsified has a low viscosity (less than 1 000 mPaxc2x7s, preferably between 1 and 1 000 mPaxc2x7s, and more preferably between 20 and 1 000 mPaxc2x7s), it is then desirable for the mixture of water and surfactant in said ratio to lead to a mixture of lamellar phase and micellar phase.
Accordingly, depending on the type of silicone used and on the nature of the surfactant, the skilled worker will easily be able to determine a range of variation of the water/surfactant ratio that is particularly favorable to the implementation of step a).
It should be understood that it is not necessary, for the implementation of this step, to form said organized phase beforehand before adding it to the oily phase, although this does correspond effectively to one preferred embodiment. In accordance with the invention, it is sufficient for the ratio of water to surfactant to be such that, were these two constituents to be mixed in this same ratio, they would form said organized phase.
It follows from this that the addition of an aqueous phase devoid of surfactant to the oily phase is another way of implementing step a), the surfactant being added simultaneously to the oily phase, or else being mixed beforehand with said oily phase.
Once the water/surfactant ratio is fixed, the skilled worker will readily determine the maximum proportion Pmax of oily phase which can be present in the emulsion. To do this it is sufficient to carry out a few preliminary tests on a laboratory scale.
One example of a test protocol is, for example, the following. It employs a mixer consisting of a frame paddle and a metal container (made, for example, of stainless steel), containing an isolating annular element (made, for example, of Teflon(copyright)), as a wall, which is arranged in the base of the container so as to allow the conductivity and hence the resistance of the solution present in the container to be measured.
Thus, when the container contains a conductive aqueous solution or an oil-in-water emulsion whose aqueous phase is conductive, the conductivity is sufficient to be measured. Conversely, when the container contains an oily phase or an oil-in-water-type emulsion, the conductivity is zero. In this way, it is easy to determine the maximum proportion of oil for which the formation of an oil-in-water emulsion is possible.
It is possible to envisage a number of methods of mixing the water, the surfactant, and the oily phase. However, the proportion Pmax depends on the chosen mixing method. Accordingly, for determining Pmax, it is appropriate to use the same addition protocol as in step a) for the addition of the aqueous phase to the oily phase. Briefly, these mixing methods are as follows:
1)xe2x80x94The oily phase, devoid of surfactant, is poured into the container and an aqueous phase consisting of water and the surfactant is added slowly (dropwise or in portions) to said oily phase. At the beginning of the addition, the conductivity is zero. The point at which a nonzero conductivity is measured allows direct determination of Pmax, Pmax representing the ratio of the amount of oily phase to the total weight of the emulsion. It is noted that the conductivity is nonzero at the point of inversion, i.e., at the point of transformation of the water-in-oil emulsion formed initially into an oil-in-water emulsion.
2)xe2x80x94This mixing method is similar to the preceding one except that the aqueous phase is added to the oily phase in one go.
3)xe2x80x94A third variant consists in proceeding as in 1) except that the oily phase poured into the mixing container contains all of the surfactant, the aqueous phase being composed exclusively of water.
4)xe2x80x94A fourth variant consists in proceeding as in 2) except that the oily phase poured into the mixing container contains all of the surfactant, the aqueous phase being composed exclusively of water.
It should be understood that, during these tests for determining Pmax, the weight ratio of the amount of water to the amount of surfactant corresponds to that set for the implementation of step a) and indicated above and the shear is maintained below 100 sxe2x88x921, for example, by setting the rotational speed of the frame paddle at around 400 rpm.
It will be noted that, in the case of highly viscous oily phases, the proportion Pmax of oil which allows a direct oil-in-water emulsion to be obtained is greater than 80%, generally greater than 90%.
Conversely, in the case of low-viscosity oily phases, the proportion Pmax of oily phase is much lower, generally greater than 30% and, for example, greater than 40%.
For the implementation of step a), the proportion of oily phase in the primary emulsion must be less than or equal to Pmax 
This proportion will be lower than the proportion of oily phase present in the final emulsion obtained at the end of the method (at the end of step b) or, where appropriate, at the end of step c)).
Generally, the primary emulsion prepared in step a) contains more than 15% by weight of oily phase, preferably more than 25% by weight, more preferably more than 30% by weight.
In one preferred embodiment, the organic phase is composed of one or more silicones.
The size of the droplets of oily phase that are dispersed in the continuous aqueous phase depends on numerous parameters and in particular on the type of organized phase that the water+ surfactant mixture makes up, on the viscosity of the oily phase and, to a lesser extent, on the chosen method of addition.
Generally, the droplets of dispersed phase have a volume-average diameter of less than 50 xcexcm, for example less than 10 xcexcm, most often less than 1 xcexcm.
When the oily phase has a high viscosity, greater than 1 000 mPaxc2x7s (preferably greater than 2 000 mPaxc2x7s), the finest particle size is obtained for a water/surfactant ratio which defines a lamellar phase.
When the oily phase has a low viscosity, less than 1 000 mPaxc2x7s (for example between 1 and 1 000 mPaxc2x7s, more preferably between 20 and 1 000 mPaxc2x7s), the finest particle size is obtained for a water/surfactant ratio which defines a two-phase mixture of lamellar phase and micellar phase.
In step b), the primary emulsion is enriched with an oily enrichment phase.
In one preferred embodiment of the invention, the oily enrichment phase has the same composition as the oily phase used in step a) for the preparation of the direct primary emulsion.
Given that the oily phase of step a) is composed preferably of one or more silicones, the oily enrichment phase will also preferably be composed of said same silicones.
The amount of oily phase used in step b) for enriching the primary emulsion must not be so excessive that it makes it impossible to prepare a direct emulsion of oil-in-water type by mixing the primary emulsion with said oily enrichment phase.
As an indication, the amount of oily enrichment phase may represent from 25 to 100% of the amount of oily phase present in the direct emulsion to be enriched, preferably from 50 to 100%, more preferably from 75 to 100%.
The protocol for mixing the primary emulsion and the oily phase under a shear of less than 100 sxe2x88x921 is not critical according to the invention. Accordingly, it is possible either to add the oily phase to the emulsion (in one go or gradually) or to add the emulsion to the oily phase (in one go or gradually).
In one preferred embodiment of the invention, mixing proceeds via the formation of an inverse emulsion of water-in-oil type and its inversion, to give, finally, a direct emulsion of oil-in-water type. In order to do this, it is possible either to add the oily phase, maintained with stirring, gradually to the emulsion or else to add the oily phase in one go to the emulsion, maintained at rest, and then to stir the combined phases present under an appropriate shear, less than 100 sxe2x88x921.
Where mixing proceeds via an inversion of the emulsion, a finer particle size is obtained in the final emulsion (lower average droplet diameter).
The effect of the enrichment step is:
to enrich the primary emulsion in organic oil;
to reduce the surfactant concentration in the final emulsion;
to narrow the particle size distribution;
to reduce the average droplet diameter of the dispersed oily phase.
Step c) makes it possible to enrich the direct emulsion subsequently by implementing step b) one or more times.
Depending on the desired oil concentration, depending on the desired surfactant concentration and depending on the desired characteristics for the particle size distribution, the skilled worker will set the required number of enrichment steps.
The emulsions obtained by implementing the method of the invention are concentrated with oil, the weight ratio of the oil to the surfactant generally being greater than 9/1 .
In the case that the final emulsion, although presenting the characteristics desired for the particle size distribution, has too high a concentration of organic oil, it is possible to dilute the final emulsion by adding an aqueous dilution phase, the technique of addition of this aqueous phase being arbitrary. The particle size of the emulsion is not modified by this dilution step. In parallel, this dilution step makes it possible to reduce further the surfactant concentration.
Accordingly, in another of its aspects, the method of the invention further comprises a step of diluting the direct emulsion obtained, by adding an aqueous dilution phase.
The method of the invention is particularly advantageous in so far as it involves neither high shear nor elevated temperatures.
The heat-sensitive direct emulsions obtained by the method of the invention, comprising one or more heat-sensitive constituents, are novel and consequently form a further subject of the invention.
As examples of emulsions, mention may be made of those comprising:
a hydroxylated polymethylsiloxane resin of type MDT having 0.5% by weight of hydroxyl groups and consisting of 62% by weight of CH3SiO3/2 units, 24% by weight of (CH3)2SiO1/2 units and 14% by weight of (CH3)3SiO1/2 units; and
a hydroxylated polymethylsiloxane resin of type DT having 2.2% by weight of hydroxyl groups and consisting of 69% by weight of CH3SiO3/2 units and 31% by weight of (CH3)2SiO2/2 units.
The method of the invention may be implemented continuously.
In this case, the mixers used are, for example, static-type mixers and the various phases are added to one another by uniting them in a single pipe of convergent flows. One particularly advantageous procedure is to convey the various flows to a point of convergence through pipes of different diameters, such that:
in step a) the flow of aqueous phase is surrounded by the flow of oily phase at the point of convergence, and
in step b) the flow of primary emulsion is surrounded by the flow of oily enrichment phase at the point of convergence.
FIG. 1 represents schematically the functional diagram of continuous implementation of a method of the invention comprising two enrichment steps.